1. Field of the Invention
The present invention relates to a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle such as an automobile or the like by use of an electric motor in correspondence to the running state of the vehicle, i.e., a turning angle of a steering wheel, etc.
2. Description of the Prior Art
Vigorous research and development has been directed to a four-wheel steering system to steer the rear wheels of a vehicle corresponding to the vehicle running state, and some systems have actually been put to practical use.
In a four-wheel steering system now in practical use, for example, as disclosed in Japanese Patent Laid-Open Publication No. 1-22575 (22675/1989), when the vehicle is running at speeds lower than a predetermined vehicle velocity, a turning ratio between the front and rear wheels is set so that a steering angle of the rear wheels becomes the reverse phase to that of the front wheels. On the other hand, when the vehicle is running at higher speeds than the predetermined vehicle velocity, the turning ratio is set so that the steering angle of the rear wheels becomes the same phase as that of the front wheels. FIG. 12 is a concrete example of the turning ratio set in the prior art four-wheel steering system.
A main advantage of the four-wheel steering vehicle is to enforce the cornering force approximately at the same time for the front wheel tires and rear wheel tires. Therefore, the four-wheel steering vehicle is allowed to move in a transverse direction faster than a front-wheel steering vehicle when the operator turns a steering wheel. In other words, if the front and rear wheels are turned in the reverse phase at low speeds and in the same phase at high speeds with an appropriate turning ratio, the vehicle starts turning at an earlier stage, thereby reducing a delay in transverse acceleration after turning of the steering wheel. This realizes, as effects in practical use, a capability of a small turn at low speeds and a capability of an emergency avoidance at high speeds, etc.
As a method to determine a steering angle of the rear wheels, the assignee of the present invention has proposed in Japanese Patent Application No. 2-212861 (212861/1990) to provide detecting means of a yaw rate (angular velocity of rotation around the center of gravity of a vehicle), so that the steering angle of the rear wheels is determined in accordance with the vehicle velocity, steering angle of the front wheels and yaw rate, and a displacement in an advancing direction of the vehicle due to external disturbances such as transverse winds or bad roads, etc. is corrected by turning the rear wheels by the determined steering angle.
However, the characteristic and response of an electric motor are varied with time in the prior art four-wheel steering apparatus of the aforementioned structure, whereby the stability of the rear wheels is undesirably endangered.
If the rotation of the electric motor is unstable, with resultant vibrations (therefore hindering the electric motor from stopping at a fixed position), the rear wheels become unsteady in their movement, resulting in an increase of power consumption in the electric motor.
Considering the yaw rate, if a signal of the yaw rate sensor is to be used for feedback, such delays as a time delay before the yaw rate is generated to the vehicle body after the rear wheels are turned, a response delay of the rear wheels, a time delay of the yaw rate sensor, a time delay for A/D conversion, a phase delay in a low pass filter to reduce noises of the yaw rate sensor, etc. make the movement of the rear wheels unstable, e.g., during the change of a lane at high speeds, thus deteriorating the steering stability.
Furthermore, a response speed to a desired steering angle of the rear wheels, which angle is determined by the angle of the steering wheel, yaw rate and vehicle velocity, namely, a response speed of the electric motor mounted in the rear wheels is set always constant in the prior art four-wheel steering apparatus, and aimed for a high speed region requiring a quick response. Therefore, the response speed is kept high even in a region not requiring a quick response, for example, even when the rear wheels remain unturned. As a result, the electric motor consumes unnecessary power.